1. Field of the Invention
The present invention relates to an image processing apparatus for bi-level-quantizing a gradated image, which is stored in digital processing, so as to record and display the bi-level-quantized image.
2. Description of the Related Art
Various copies are processed in conventional image processing apparatuses such as scanner, copying machine and facsimile. In a binary image region such as letters, for example, an image is quantized into bi-level with a fixed threshold so as to make the outline of the image clear, while in a continuous-tone image region such as a photograph a dither threshold matrix processing is adopted in order to enhance the gradation reproducibility.
In a pseudo halftone region such as screened-halftone, it is necessary to subject the region to a dither threshold matrix processing after it is filtered through a screened-halftone which suppresses the screened-halftone components in order to prevent moires.
Many copies such as a catalog have a binary image region such as letters and a continuous-tone image region in the form of a mixture. In this case, it is necessary to distinguish each region and to subject each region to an appropriate processing before outputting it.
As such processings, Japanese Patent Laid-Open No. Sho 58-3374 discloses a method of distinguishing a binary image region from a continuous-tone image region by obtaining the maximum luminance and the minimum luminance of the pixels in a block, Japanese Patent Laid-Open No. Sho 64-57877 discloses a method of distinguishing regions in screened-halftone printing by obtaining the difference in an average luminance of the pixels in a plurality of surrounding blocks, and Japanese Patent Laid-Open No. Hei 2-112077 discloses a method of distinguishing the regions on the basis of the number of pixels which constitute the peak or the trough of the curve of a gradation change in a block.
Japanese Patent Laid-Open Nos. Sho 64-57877, Hei 2-112077 and Hei 2-115987 disclose a technique of detecting screened-halftone pattern pixels by using information on the regions surrounding the target pixel.
None of these conventional methods, however, can realize the ideal discrimination of the regions, and greatly deteriorate the picture quality on the boundary of different regions at the time of bi-level-quantizing. For example, the portion having a high resolution frequency component such as an eye and hair in a portrait is judged to be a binary image region because the difference in the maximum luminance and the minimum luminance in the decision block containing that portion becomes locally large. As a result, in the output binary image, a bi-level-quantized image portion with a fixed threshold with an intensified contrast suddenly appears in a smooth pseudo halftone image.
In screened-halftone printing, in the portion in which the luminance level greatly changes such as the portion at which a gray background changes into black hair, since the difference in the average luminance of the pixels in the blocks in the vicinity of the target pixel is large, the portion is judged to be a binary image portion. Consequently, in the output image, the black hair portion which is processed by bi-level-quantizing with a fixed threshold appears in a flat background image which is smoothed and pseudo-halftoned by the filter, thereby impairing the smoothness of the image.